The "Display Emulation" option works like this: when selected, it tries
to load "<path>/Video Shaders/Emulation/<systemName>.shader/"; otherwise
it falls back to the blur shader. <path> is the usual (next to binary,
then in <config>/higan, then in /usr/share/higan, etc); and <systemName>
is "Famicom", "Super Famicom", "Game Boy", "Game Boy Color", "Game Boy
Advance"

To support BSD, I had to modify the $(platform) variable to
differentiate between Linux and BSD.
As such, the new $(platform) values are:
win -> windows
osx -> macosx
x -> linux or bsd

I am also checking uname -s instead of uname -a now. No reason to
potentially match the hostname to the wrong OS type.